1. Field of Invention
The present invention relates to a power converter, and more specifically relates to a control circuit of the power converter.
2. Description of the Related Art
The primary side regulation technologies had been disclosed in many prior arts such as, “PWM controller regulating output voltage and output current in primary side”, U.S. Pat. No. 6,721,192; “Primary-side controlled flyback power converter”, U.S. Pat. No. 6,853,563; “Close-loop PWM controller for primary-side controlled power converters”, U.S. Pat. No. 7,016,204; and “Switching control circuit having off-time modulation to improve efficiency of primary-side controlled power supply”, U.S. Pat. No. 7,362,593; etc. The drawback of these prior arts is the slow response to the change of the output load, particularly when the power converter is operated in light load or no load.
FIG. 1 is a circuit schematic of a prior art of a primary side regulation power converter. A transformer 10 has a primary winding NP, a secondary winding NS, and an auxiliary winding NA. A terminal of the primary winding NP is coupled to an input terminal of the power converter to receive an input voltage VIN. The secondary winding NS generates an output voltage VO at an output terminal of the primary side regulation power converter via a diode 40 and a capacitor 45. A transistor 20 is coupled to the other terminal of the primary winding NP to switch the transformer 10 for transferring the energy from the input terminal to the output terminal of the power converter. When the transistor 20 is turned on, the transformer 10 is magnetized. The transformer 10 is demagnetized and the energy of the transformer 10 is delivered to the capacitor 45 via the diode 40 for generating the output voltage VO at the output terminal once the transistor 20 is turned off. Meanwhile, a reflected voltage VAUX is generated at the auxiliary winding NA of the transformer 10. The secondary winding NS is proportional to the primary winding NP. The output voltage VO is thus correlated to the input voltage VIN.
A voltage divider developed by resistors 51 and 52 is coupled to the auxiliary winding NA for generating a reflected signal VS in response to the reflected voltage VAUX of the auxiliary winding NA. The reflected signal VS is thus correlated to the reflected voltage VAUX. The auxiliary winding NA is proportional to the primary winding N. The reflected voltage VAUX is thus correlated to the output voltage VO during the period that the transformer 10 is demagnetizing. In other words, the reflected signal VS is also correlated to the output voltage VO. A control circuit 50 is coupled to sample the reflected signal VS of the voltage divider for generating a switching signal SW. The control circuit 50 is further coupled to receive a current-sense signal VCS for adjusting the switching signal SW. The switching signal SW controls the transistor 20 to switch the transformer 10 and regulate the output voltage VO.
A current-sense device 30, such as a resistor, is coupled between the transistor 20 and the ground. The current-sense device 30 senses a switching current IP of the transformer 10 and generates the current-sense signal VCS in response to the switching current IP.
The reflected signal VS is correlated to the output voltage VO during the period that the transformer 10 is demagnetizing. Therefore, the information of the output voltage VO can only be sampled when the transformer 10 is switched on/off. Because the switching frequency of the switching signal SW is decreased during the light load condition or the no load condition for reducing the power loss of the power converter, the information of the output voltage VO can not be detected in between the switching of the transformer 10. Thus, a significant voltage drop of the output voltage VO would be happened when the output load of the power converter is increased rapidly from the light load to the heavy load.
FIG. 2 shows the waveforms of the switching signal SW, the output load LOAD and the output voltage VO of the primary side regulation power converter shown in FIG. 1. The level of the output voltage VO is decreased significantly when the output load is increased suddenly during the light load condition. In the light load condition, the switching period of the switching signal SW is long for reducing the switching loss of the power converter. Because the response of the power converter is slow to the change of the output load, the significant voltage drop ΔVO1 of the output voltage VO would be happened when the output load of the power converter is increased rapidly from the light load to the heavy load.